Since whey protein is high in nutrition value and protein utilization efficiency as compared with casein and soybean protein, it is known that whey protein is used as a substituent for breast milk or a protein source for nutritive composition. In particular, .beta.-lactoglobulin (hereinafter referred to as ".beta.-Lg"), which is a main component of whey protein in milk, is not present in breast milk and acts as an allergen for infantile allergy. Therefore, when whey protein is used as a substituent for breast milk, it is recommended that .beta.-Lg be reduced or a material having a high content of .alpha.-lactalbmin (hereinafter referred to as ".alpha.-La") be used.
Up to this date, whey, a by-product in the manufacture of cheese or casein, is used in food as it is or in various forms, for example, in the form of low lactose whey wherein lactose is removed from whey, in the form of desalted whey produced by treating whey with various desalting devices, or in the form of whey protein concentrated (hereinafter referred to as "WPC") produced by treating whey with ultrafilters. On the other hand, as a process for fractionating whey protein contained in whey into individual components, there has been proposed a process for reducing .beta.-Lg or a process for manufacturing a high .alpha.-La content composition.
As a process for separating and recovering a high .alpha.-La content fraction, there have been proposed various processes such as those described by Kuwata et al. (J. Food Sci., 50 (1985), R. J. Pearce (Aust. J. Dairy Technol., 42 (1987) and J. L. Maubois et al. (Japanese Unexamined Patent Published Application (hereinafter referred to as "J. P. Kokai") Sho No. 56-36494). These processes use whey as a starting material and utilize the difference in physical and/or chemical properties of various kinds of whey proteins. However, these processes have various drawbacks in that their steps are complicated; they are high energy consuming; they have low recovery; and they cause irreversible change of proteins. Therefore, they have not been accepted as an industrially practicable method.
As a process for recovering whey protein at a high concentration, there have been proposed processes using an ion exchanger as described in J. N. de Wit et al. (Neth. Milk Dairy J. 40 (1986)) and J. S. Ayers et al. (New Zealand J. Dairy Sci. and Tech., 21 (1986)). However, in these processes, at most the solution, which has passed through an ion exchanger (hereinafter referred to as "exchanger-passed solution") as a by-product, is merely utilized for the production of lactose and there has not been proposed processes for highly utilizing whey protein contained in the ion exchanger-passed solution.